Drilling operations play an important role when developing oil, gas or water wells or when mining for minerals and the like. A drilling fluid such as drilling mud is typically injected into a wellbore when performing drilling operations. The drilling fluid may for example be water, a water-based mud, an oil-based mud, or another drilling fluid. During the drilling operations, a drill bit passes through various layers of earth strata as it descends to a desired depth. Drilling fluids are commonly employed during the drilling operations and perform several important functions including, but not limited to, removing the cuttings from the well to the surface, controlling formation pressures, sealing permeable formations, minimizing formation damage, and cooling and lubricating the drill bit.
Dual gradient drilling systems may provide significant advantages over conventional fluid circulatory systems, particularly in undersea drilling applications. As noted, drilling fluids used in drilling a well may provide pressure in the open wellbore in order to prevent the influx of fluid from the formation. Thus, the pressure in the open wellbore is typically maintained at a higher pressure than the fluid pressure in the formation (the pore pressure). On the other hand, drilling fluid circulation also typically is controlled so as to be below the fracture pressure, the point at which a formation fracture can occur (the fracture pressure). Once the formation fractures, returns flowing in the annulus may exit the open wellbore thereby decreasing the fluid column in the well. If this fluid is not replaced, the wellbore pressure can drop and allow formation fluids to enter the wellbore, causing a kick and potentially a blowout. Thus, within a formation, it may be desirable to circulate drilling fluids in the well such that well pressure is maintained between the pore pressure and the fracture pressure.
This system may be complicated in undersea drilling applications, particularly deep sea drilling applications wherein the allowable pressure gradient of formations below the mudline may be significantly reduced as compared to conventional drilling operations due to the resulting difference in overburden due to seawater as compared to conventional rock formations. At the same time, pressure in the casing above the mudline (through the seawater) must also be maintained such that seawater does not breach the casing.
Accordingly, dual gradient drilling systems may be used to isolate the borehole pressure gradient below the mudline or sea floor from the drilling mud pressure gradient above (that is, in the casing through the seawater). Whereas single gradient drilling technology seeks to control wellbore pressure using a column of substantially constant-density drilling fluid from the bottom of the well back to the rig, dual-gradient drilling may use a lower density fluid, in some instances about the same density as seawater, from the rig to the seafloor, and then uses a heavier density drilling fluid below the mudline—that is, within the actual formation, between the seafloor and the bottom of the well. Dual-gradient drilling techniques may, in effect, simulate the drilling rig being located on the seafloor and therefore avoid some of the problems associated with deep-water drilling.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.